MDL-22478 Allow alternative location of strings disk cache

Administrators can now specify $CFG->langcacheroot in their config.php for
alternative location of strings disk cache. If the alternative location is
not set, Moodle uses default $CFG->dataroot/cache/lang

config-dist.php

@@ -345,6 +345,18 @@
 //
 //     $CFG->themedir = '/location/of/extra/themes';
 //
+// If $CFG->langstringcache is enabled (which should always be in production
+// environment), Moodle keeps aggregated strings in its own internal format
+// optimised for performance. By default, this on-disk cache is created in
+// $CFG->dataroot/cache/lang. In cluster environment, you may wish to specify
+// an alternative location of this cache so that each web server in the cluster
+// uses its own local cache and does not need to access the shared dataroot.
+// Make sure that the web server process has write permission to this location
+// and that it has permission to remove the folder, too (so that the cache can
+// be pruned).
+//
+//     $CFG->langcacheroot = '/var/www/moodle/htdocs/altcache/lang';
+//
 // Site default language can be set via standard administration interface. If you
 // want to have initial error messages for eventual database connection problems
 // localized too, you have to set your language code here.

lib/moodlelib.php

@@ -5484,12 +5484,17 @@ function get_string_manager($forcereload=false) {
     }
     if ($singleton === null) {
         if (empty($CFG->early_install_lang)) {
+            if (empty($CFG->langcacheroot)) {
+                $langcacheroot = $CFG->dataroot . '/cache/lang';
+            } else {
+                $langcacheroot = $CFG->langcacheroot;
+            }
             if (empty($CFG->langlist)) {
                  $translist = array();
             } else {
                 $translist = explode(',', $CFG->langlist);
             }
-            $singleton = new core_string_manager($CFG->langotherroot, $CFG->langlocalroot, "$CFG->dataroot/cache/lang", !empty($CFG->langstringcache), $translist);
+            $singleton = new core_string_manager($CFG->langotherroot, $CFG->langlocalroot, $langcacheroot, !empty($CFG->langstringcache), $translist);
         } else {
             $singleton = new install_string_manager();
         }